1. Field of the Invention
The present invention relates to the improvements of a reciprocating piston of an internal combustion engine suitable for automotive vehicles.
2. Description of the Prior Art
In reciprocating pistons used for automotive internal combustion engines, during reciprocating motion of the piston, the piston operates with the piston crown or piston head exposed to hot combustion gases, whereas the piston skirt contacts the comparatively cool cylinder wall. This results in a temperature gradient or difference from the top of the piston to the bottom. Generally, the temperature of the piston top exposed to the combustion chamber is higher than that of the piston bottom. Thus, there is a difference of thermal expansion from the top of the piston to the bottom. In order to control the effects of differential thermal expansion between the top and bottom portions of the piston and to absorb a relatively great thermal expansion of the top of the piston, the top portion of the piston is formed with a plurality of lands for example a top land, a second land, and a third land, so that there is a desired piston land-to-cylinder wall clearance between each of the lands and the cylinder wall. The desired piston land-to-cylinder wall clearance will be hereinafter referred to as a "thermal-expansion control clearance" or a "thermal-deflection control piston-to-cylinder clearance". The thermal-expansion control clearance contributes to absorption of the relatively great thermal expansion which takes place at the upper portion of the piston. Additionally, a plurality of piston rings, such as a top compression ring and a second compression ring, are fitted respectively to a ring groove defined between the top and second lands and a ring groove defined between the second and third lands, for effective sealing on the power stroke. One such reciprocating-piston structure of an automotive engine has been disclosed in Japanese Patent Provisional Publication No. 6-101566. As to the profile of the respective piston land and the profile of the piston skirt of the conventional reciprocating piston structure disclosed in the Japanese Patent provisional Publication No. 6-101566, the profile of upper and lower portions of the piston skirt is slightly inwardly curved to maintain an adequate oil film on the cylinder wall, and also at least the top land and the second land are designed to have respective curved tapers being continuous from the upper portion of the piston skirt to stabilize the behavior or reciprocating motion of the piston during operation of the engine. In other words, the thermal-expansion control clearance (the piston top-to-cylinder wall clearance) is designed to be relatively excessive. Thus, there is a tendency for some of fuel condensed to liquid droplets to remain in the thermal-expansion control clearance, thus resulting in deposits of hydrocarbon on piston rings, valves, and other engine parts, or in the piston ring grooves or beneath the piston crown. The relatively excessive thermal-expansion control clearance also results in increased oil consumption. To the contrary, if the thermal-expansion control clearance is insufficient, it is impossible to satisfactorily absorb the comparatively great thermal expansion of the top of the piston, and thus there is undesiredly increased friction between the cylinder wall and the top of the piston. Seizure of the piston in the cylinder could occur owing to thermal expansion insufficiently controlled. As discussed above, the conventional piston structure disclosed in the Japanese Patent provisional Publication No. 6-101566, has a relatively excessive piston land-to-cylinder wall clearance (or a relatively excessive thermal-expansion control clearance). In order to satisfy a necessary sealing performance of this clearance, and to prevent combustion pressure from escaping from the combustion chamber and to adjust the film of lubricating oil on the cylinder wall, the previously-noted conventional piston structure often uses a labyrinth seal. As is generally known, the labyrinth seal structure requires a plurality of piston rings fitted to the respective piston grooves, thereby resulting in the increase in total weight of the piston and also increasing an amount of frictional resistance during reciprocating motion of the piston. Additionally, in the piston of the curved profile or curved taper of the upper portion of the piston, provided for thermal expansion control at the top of the piston, the piston diameter is diametrically diminished from the piston-skirt upper portion (or the third land) to the top land, and thus there is an increased tendency for the piston slapping noise-reduction performance to be lowered.